This invention is directed to a method of treating cold-worked zirconium alloys to reduce large grain growth during thermal treatment above the recrystallization temperature of the alloys. In particular, the method of the present invention is directed to desensitizing cold-worked zirconium alloys (e.g., zircaloy-2 or zircaloy-4) to large grain growth during thermal treatment above the recrystallization temperature of the alloy. The desensitized alloys have a specific utility in the fabrication of nuclear reactor fuel elements.
Zirconium based alloys such as zircaloy-2 and zircaloy-4 are commonly used in nuclear reactor fuel element cladding. Among the problems encountered in manufacturing fuel elements from zirconium-based alloys is that the alloys are susceptible to large grain growth (LGG). It has been observed that zirconium based alloys which have experienced cold plastic deformation in the range of about 2 to 8 percent strain, undergo LGG during thermal treatment of the alloy at or above the recrystallization temperature. LGG often results in a fuel element which is undesirable for reactor use because the large grains degrade the mechanical properties and possibly the corrosion resistance of the cladding.
A common approach to avoiding LGG is to assure that any zircaloy experiencing thermal cycles in the recrystallization temperature range does not experience cold working in the critical range of about 2 to 8 percent prior to the thermal treatment. In cases where prior strains are inevitable or where cold working in the critical range occurs during the thermal treatment (i.e., on the way up to the recrystallization temperature), prestraining the material to a level well above the critical range has been used. However, since prestraining can degrade machinability, increase cost or be impractical for some geometries, this process is not always available. The process of the present invention is proposed as an economical and viable alternative to the prestraining treatment.